Add reference to indexer Pid to task status

When we run re-compaction stage we start second index updater from
view compactor. In this case couch_task_status:all would return two
identical entries for the same `design_document` and `shard`. Which is
very confusing. Add reference to indexer Pid to disambiguate them.
For the recompaction case the indexer_pid will be pointing to compaction task.
diff --git a/src/couch_mrview_updater.erl b/src/couch_mrview_updater.erl
index 8a85887..645f02b 100644
--- a/src/couch_mrview_updater.erl
+++ b/src/couch_mrview_updater.erl
@@ -38,6 +38,7 @@
     Self = self(),
     MapFun = fun() ->
         couch_task_status:add_task([
+            {indexer_pid, Partial},
             {type, indexer},
             {database, State#mrst.db_name},
             {design_document, State#mrst.idx_name},